1. Field of the Invention
The present invention relates to an optical disc for storage and retrieval of digital data and to a method of protecting the optical disc.
2. Discussion
Optical discs, such as compact discs (CDs) and digital videodiscs (DVDs), have become the accepted medium for storing and retrieving large amounts of digital information (data). Standard CDs and DVDs have the same physical dimensions (12 cm OD, 1.2 mm thickness), but differ primarily in the amount of data that each can hold. A standard CD can store up to about 783 megabytes of audio programming, while DVDs can hold between about 4.38 gigabytes (single-sided/single-layer DVD) and about 15.9 gigabytes (double-sided/dual-layer DVD) of multimedia programming (photographs, video, audio, etc.). Other optical storage discs include read only memory compact discs (CD-ROMs), recordable compact discs (CD-R), recordable DVDs (DVD-R), and rewritable compact discs (CD-RW). Though physically similar to audio CDs, CD-ROMs, CD-Rs and CD-RWs can store slightly less data (i.e., less than about 700 megabytes) because a fraction of their respective storage capacities are used by a file system and data associated with enhanced error correction.
Optical discs owe their large storage capacity to the way they represent digital data. With CDs and single-layer DVDs, digitized (binary) data are encoded on the discs as a sequence of microscopic pits separated by smooth areas (lands) that define a continuous track that spirals outward from the center of the disc. Adjacent tracks on CDs are 1600 nm apart, and the minimum pit length is 830 nm. DVD's achieve their greater storage capacity, in part, by shrinking the distance between adjacent tracks (740 nm) and by decreasing the minimum pit length (400–440 nm). Recordable compact discs and rewritable compact discs employ similar data encoding, except that the “pits” on CD-Rs and CD-RWs are replaced by “dark” spots formed, respectively, on a light-sensitive organic dye layer or light-excitable crystal layer.
Optical disc readers (CD or DVD players, CD-ROM, CD-R or CR-RW drives, etc.) retrieve data using a laser pickup assembly and a tracking system. During playback, the laser pickup assembly focuses a laser beam on the spinning optical disc, while the tracking system moves the laser pickup assembly outward from the center of the disc. The optical reader adjusts the angular speed of the disc during data retrieval so that pits and lands of a single track stream past the laser beam at constant linear velocity. The optical pickup includes a detector (e.g., photodiode array) which detects any light reflected by the optical disc. Laser light hitting a land reflects at a higher intensity than laser light hitting a pit (or dark spot) which scatters the light. The optical disc reader translates these temporal changes in detected light intensity into a stream of binary data.
Optical discs have relatively simple, but elegant construction. Digital videodiscs, for example, are composed of one or more layers of plastic (e.g., optical grade polycarbonate) that are individually formed by injection molding. One surface of each layer contains the encoded data as a spiral track of microscopic pits and lands, while another surface is substantially planar. Prior to assembling the layers, DVD manufactures cover the surface containing the pits and lands with a thin metallic layer. The plastic layers that will become the outermost layers of the DVD are coated with semi-reflective gold, while the plastic layers that will become the innermost layers are coated with aluminum. The use of gold allows the laser pickup assembly to focus laser light through the outer layers onto the inner layers of the DVD. Following preparation of the plastic layers, each is coated with acrylic lacquer, pressed together, and cured to form the disc. For single-sided discs, a label is applied onto the non-readable side (i.e., side opposite the polycarbonate layer or layers containing pits and lands). Audio CD and CD-ROMs are made in a similar manner, but comprise a single polycarbonate layer laminated to a metallic film and relatively thin acrylic layer.
Compared to competing technologies such as magnetic storage media, optical discs are mechanically robust and inexpensive. Despite these advantages, however, optical discs can be improved. Although the polycarbonate plastic layer has excellent optical properties and good dimensional stability, it can be scratched during handling, which may compromise data stored on the disc. For example, after removing compact discs from their protective cases, users often place them on comparatively hard flat surfaces, such as a tabletop or desktop, with the polycarbonate or readable side face down (label-side face up). Since optical discs are quite thin, users find it difficult to pickup CDs without dragging them across the tabletop. In doing so, hard contaminants on the surface of the tabletop and any defects in the tabletop surface may scratch, gouge, or scuff the polycarbonate plastic layer. Similarly, users often stack CDs to conserve space. Any dirt particles trapped between individual CDs may also damage the surfaces of individual CDs during handling of the stack. Although the optical properties of polycarbonate and on-disc error correction help reduce the affects of surface scratches, repeated damage to CD surfaces over time may render some data unreadable.
The present invention overcomes, or at least mitigates, one or more of the problems described above.